The casting of molten metal and particularly molten metals such as molten lead for battery parts is done under different casting conditions. One of the casting methods is high pressure intensification which involves increasing the pressure of molten lead in the cavity by driving a piston into the molten metal to substantially increase the pressure. This process of intensification is described more fully in Ratte U.S. Pat. Nos. 6,202,733; 6,363,996; 6,405,786; 6,499,530; 6,513,570; 6,598,658 and 6,564,853 and uses pressures that compress the metal to reduce the volume of air bubbles in the metal.
Another method of casting battery parts is gravity casting. Gravity casting is preferred for casting larger parts that cool slowly because the gravity casting allows the molten metal to slowly flow under the pressure of gravity to fill any voids in mold cavity as the molten metal solidifies. This results in a part that is substantially free of cracks and voids. Gravity casting uses the head pressure generated by the molten metal to fill out the mold cavity. Thus gravity casting is done at a low fluid pressure within the molten metal. In certain applications, such as larger parts that are immersed in an acid, a gravity cast battery part is highly desirable since the molten metal flows and fills during the solidification process thus virtually eliminating solidification cracks and stresses in the battery part. Since cracks and stresses in a battery part, which is immersed in an acid, can cause rapid deterioration of the battery part it is generally preferred to gravity cast large articles if the article is located in an acid such as found in a battery. However, one of the disadvantages of gravity cast articles is that the articles generally lack the surface definition of high pressure injection molded parts.
The present invention provides an enhanced gravity casting process wherein the molten metal is allowed to solidify under gravity casting conditions while at the same time a follower, which is a portion of the mold surface, is maintained under a following pressure to follow the volume contraction of the molten metal as the molten metal solidifies. That is, as the metal shrinks during solidification the pressure on the molten metal is maintained so that the mold surface or follower moves toward the mold cavity in response to the shrinkage due to solidification. A further feature of the invention is that at the same time air is allowed to escape from the molten metal through a passage which is sufficiently small that molten lead does not flow therepast.